Move benchmark/simulate-traits to UnfoldSim.jl?

[behinger]

Judith and I discussed to combine this: https://github.com/unfoldtoolbox/UnfoldStats.jl/blob/extract_coefs-function/benchmark/sim_and_fit.jl with this: https://github.com/unfoldtoolbox/UnfoldSim.jl/blob/main/src/predefinedSimulations.jl

Benefits would be:

• simpler predef-modifications of the predef_eeg output, e.g. using MultipleEventTypes, SinglePredictor vs. ManyPredictors, Multichannel

Negative

• we have to reimplement some things and think
• the "fitting" part is only interesting for UnfoldStats